1. Field of the Invention
This invention relates to a display device using electrochromic (EC) materials exhibiting a reversible color change taken place as a result of an electrochemical redox reaction and to a method of driving the display device.
2. Prior Art
In a display device in which a film of an EC material such as tungsten oxide is arranged in the form of a plurality of segments, the coloration density of each of the segments is determined by the quantity of electric charge injected therein. In order to drive these display segments at all times with a uniform coloration density, it is necessary to maintain constant the quantity of electric charge to be transferred. Since the conductivities of EC materials and electrolytes vary with ambient temperatures, a complicated temperature compensating circuit is required to realize a constant electric charge drive.
As means for avoiding this difficulty, Tabata et al, in U.K. Patent Application GB 2,083,261A have disclosed a driving method called a charge transfer method together with a liquid type display device using WO.sub.3 as the EC material. With this display device, a voltage is applied between the colored segment and the colorless segment and the electric charge is transferrd from one segment to another. After the electric charge previously injected in one segment has been transferred to another segment, current flow is naturally stopped. In this way, a constant electric charge drive is assured independent of ambient temperature.
As the electrolyte, however, their display device uses a liquid electrolyte which causes leakage and thus corrosion in the electrode film. In order to eliminate these problems, a solid electrolyte such as an ion permeable solid insulator may be used in place of the liquid electrolyte. But this will create new problems such as delay in the response times of the device, poor coloration density, and degradation of the device owing to the evolution of hydrogen gas.